Capacitive sensors have recently become increasingly common finding application on a variety of devices for providing touch sensitive switches for activating functions. Capacitive position sensors also find use in non-interface applications, for example detecting fluid levels.
FIG. 1 schematically shows a known type of capacitive sensor 2 which may be operated in accordance with the charge transfer techniques described in U.S. Pat. No. 5,730,165 and/or U.S. Pat. No. 6,466,036. The sensor is for measuring the capacitance Cx of a sense electrode 4 to a system reference potential (ground). The capacitance provided by the sense electrode to the system ground potential may thus be considered as being equivalent to a capacitor having capacitance Cx with one terminal connected to ground, and may be referred to as a sense capacitor.
The sensor 2 may be arranged to detect the presence of a pointing finger 8 adjacent the sense electrode 4, although in other examples the sensor 2 may be used to detect a substance such as a fluid which in some way introduces a grounding effect on the sense electrode 4. When there is no finger adjacent the sense electrode 4 the capacitance formed thereby to ground is relatively small. When there is a finger adjacent the sense electrode 4 (as in FIG. 1), the sense electrode's capacitance to ground is increased as the pointing object provides a capacitive coupling Cx to a virtual ground. Thus changes in the measured capacitance of the sense electrode are indicative of changes in the presence of an adjacent object (e.g. a finger in a touch sensitive control, or a fluid in a level sensor). The sensor of FIG. 1 is a single-channel sensor in that it is operable to measure the capacitance of a single sense electrode 4.
In addition to the sense electrode 4, the sensor 2 comprises a controller 6 and a sample capacitor Cs. The controller 6 is a general purpose programmable device configured to perform a sequence of functions, which control the operation of a biasing arrangement which includes a set of switches, to generate a measure of the capacitance of the sensing electrode Cx to a system reference or ground.
Two pins P1, P2 of the controller 6 may be driven high or low by the biasing arrangement within the controller, in a defined sequence, as the controller executes an operating program. This defined sequence is schematically represented in FIG. 1 by the operation of a series of switches S1, S2, S3, which form the biasing arrangement within the controller 6. Switch S1 selectively connects pin P1 to a logic level+V, which drives the pin P1 high. Switch S2 selectively connects pin P1 to the controller's system reference potential (ground), which corresponds to the action of driving pin P1 low. Only one or other (or neither) of S1 and S2 can be closed at any one instant. Switch S3 similarly drives pin P2 high or floats P2.
The controller is also operable to provide a measurement circuit M connected to pin P1 (i.e. pin P1 is an I/O pin), in addition to being arranged to configure switches S1 and S2 to driven pin P1 high and low. In one example the measurement circuit includes a simple comparator arranged to compare an input voltage on pin P1 with a threshold level Mthresh. Typically, the threshold level might be half the controller's operating voltage (i.e. Mthresh=+V/2).
The sample capacitor Cs is connected between pins P1 and P2. The sensing electrode 4 is connected to pin P2.
FIG. 2 shows a table which schematically represents a switch operating sequence of the biasing arrangement of the controller 6 for the touch sensor of FIG. 1 for measuring the capacitance of the sensing electrode 4 to system ground. The sequence operates in a series of steps starting at step 1, as indicated in the left-hand column. The columns headed S1 to S3 indicate the status of the respective switches in each step. An “X” in the table indicates the corresponding switch is closed, while an “O” indicates the corresponding switch is open. The columns headed P1 and P2 indicate the voltage level of the corresponding pins at each step. A table entry “LOW” indicates the corresponding pin is driven low, a table entry “HIGH” indicates the corresponding pin is driven high, where the pin is not driven high or low, its “free” voltage level is indicated. The final column provides brief comments on the step.
Step 1 is an initialisation/reset step. Switches S2 and S4 are closed so that pins P1 and P2 are both driven low. This in effect grounds the sense electrode 4 and shorts out the sample capacitor Cs so that there is no charge residing on either.
Step 2 is a charging step in which only switch S1 is closed. Thus pin P1 is driven high while pin P2 is free to float. The voltage +V provided on pin P1 thus charges the in-series combination of the sample capacitor Cs and the sense capacitor Cx.
The sample capacitor Cs and the sense capacitor Cx provide a capacitive divider between +V and ground. The voltage on pin P2 at the capacitors' common connection is the voltage across Cx (i.e. V(Cx)). This depends on the relative capacitances Cs and Cx. I.e. V(Cx)=V*Cs/(Cs+Cx) in accordance with the well-known capacitor divider relationship. The voltage across the sample capacitor Cs is V(Cs) where V(Cs)=V·V(Cx). I.e. V(Cs)=V*(Cx/(Cs+Cx)).
Step 3 is a measuring step in which only switch S3 is closed. Thus pin P1 is free to float and pin P2 is driven low. Driving pin P2 low means (i) the charge on Cx is removed (sunk to ground), and (ii) pin P1 achieves the voltage V(Cs) established across the sample capacitor Cs during the charging step 2. Thus the voltage on pin P1 is V(Cs)=V*(Cx/(Cs+Cx)). The voltage on P1 thus depends on the capacitance of the sense capacitor Cx provided by the sense electrode 4. In principle, this voltage may be measured to provide an indication of the capacitance of the sense capacitor. However, in practice the voltage V(Cs) provided by the single charging cycle in step 2 will be small, because the capacitance of the sensing electrode Cx will be very much less than the capacitance of the sampling capacitance Cs. Thus to provide a more robust measure of capacitance Cx the senor 2 is operable to repeatedly execute steps 2 and 3 (i.e. without performing the reset step 1). In each repetition of steps 2 and 3 a discrete increment of charge is added to the sample capacitor. Thus the voltage V(Cs) after each iteration of steps 2 and 3 increases asymptotically in dependence on the magnitude of the sense capacitor Cx. The increase is asymptotic because less charge is added in subsequent iterations because of the charge already on the sample capacitor Cs.
After a number of these charge cycles (i.e. a burst of pulses), the voltage on pin P1 may be measured and taken as an indicator of Cx. However, this requires the measurement channel M of the controller 6 which is associated with pin P1 to have the capability of measuring an analogue voltage. This requires relatively complex circuitry. Thus it is common not to burst for a fixed number of pulses, but to simply keep bursting (i.e. iterating steps 2 and 3) until the voltage V(Cs) reaches a measurement threshold Mthresh, e.g. where typically Mthresh=V/2. The number of charging cycles required for the voltage across the sample capacitor Cs to exceed the measurement threshold (as determined by a simple comparator) is an (inverse) measurement of the capacitance to ground of the sense electrode, and hence indicative of the proximity or otherwise of an object. The “variable burst length” scheme has the advantage over “fixed burst length” schemes of using a comparator instead of a more complex voltage measurement function. Nevertheless, both schemes are found to provide robust and reliable measurements of relatively small capacitances.
Although the touch sensor illustrated in FIGS. 1 and 2 operate efficiently to detect the presence of an object, such as a finger, adjacent the sense electrode, there are some circumstances, where two or more such touch sensors may operate in close proximity, which can cause a false detection of an object to be induced.